This invention relates to implantable articles and methods for manufacturing such articles. More particularly, the invention relates to bone prosthesis and process for manufacturing the same.
There are known to exist many designs for and methods for manufacturing implantable articles, such as bone prostheses. Such bone prostheses include components of artificial joints, such as elbows, hips, knees and shoulders. An important consideration in the design and manufacture of virtually any implantable bone prosthesis is that the bone prosthesis has adequate fixation when implanted within the body.
Early designs of implantable articles have relied upon the use of cements such as polymethylmethacrylate (PMMA) to anchor the implant. The use of such cements can have some advantages, such as providing a fixation that does not develop free play or does not lead to erosion of the joining faces postoperatively. Maintaining a load or force at the cement bone interface assists in providing for good fixation and to prevent motion.
To assist in maintaining the load at the cement bone interface tapered, some highly polished stems have been designed without a proximal collar to permit the subsidence within the cement mantle. The stems are thus permitted to move distally with respect to the resected bone. Long term controlled subsidence within the cement mantle minimizes cement abrasion.
Without a collar, however, the surgeon is intra-operatively challenged to position these stems both axially and rotationally. A less than ideal position of the stem within the bone, also known as malposition, has been shown to limit the patients range of motion by inducing improper leg length, inadequate lateral stem offset or non-anatomical version of the stem.
Inadequate pressurization of the cement within the femoral canal has also been documented as a potential cause of improper cement technique. Centralization of the stem within the cement mantle is also critical for successful results. Non-uniform or excessively thin cement/stem/bone interfaces may lead to high internal stresses and subsequent cracks. Cement debris generation due to abrasion has also been shown to produce excessive third-body wear of the polyethylene acetabular components as well as potentially induce osteolytic reactions and bone resorptions that may lead to stem loosening. One single femoral stem that can singly address these critical issues is the intended solution.
The proper distribution of stresses within the prosthesis and throughout the surrounding bone is a problem in the use of known hip joint systems. If too little stress is applied to the bone, resorption can occur leading to atrophy of the affected area. Too much stress may result in an undesirable hypertrophy of the affected area. In some prior art, femoral stem designs' excess forces are transmitted through the relatively rigid stem to the distal portion, resulting in hypertrophy of the bone surrounding the distal portion, and atrophy of the bone surrounding the proximal portion of the stem.
Attempts have been made to provide for a proper amount of stress on the cement mantle of prosthesis. For example, in U.S. Pat. Nos. 5,171,275 and 5,290,318 both to Ling, et al, incorporated herein by reference, disclose a tapered, collarless femoral hip joint prosthesis formed of cobalt chromium-molybdenum alloy with a highly polished surface. The stem is tapered in the anterior/posterior and medial/lateral directions and has rounded corners.
The tapered collarless design permits the polished stem to subside within the cement mantle. The taper of the stem permits it to self-tighten upon the slight movement which occurs during the subsidence and engage in the hollow centralizer and yet to do so without pulling the cement mantle and avoiding the disruption of the micro interlocking at the cement bone interface.
This design causes the stem to impart primary compressive forces against the cement mantle thus transmitting the load to the femur. Transmitting the load in this manner forces the cement mantle continuously, snuggly, and firmly against the interior of the femur to assist in maintaining the integrity of microlocking at the cement bone interface.
Utilizing devices such as those shown in Ling without a collar, however, the surgeon is intra-operatively challenged to position the stems both axially and rotationally. The inability to properly position the prosthesis may limit the patient's range of motion by inducing, for example, improper leg length, inadequate lateral stem offset or none anatomical version of the stem. Conversely the inclusion of a collar on the femoral hip joint prosthesis may lessen or eliminate the ability of the femoral stem prosthesis to subside and impart the primary compressive forces against the cement mantle necessary to assure that the cement mantle continuously is snugly and firmly against the interior of the femur to maintain the integrity of the microlocking at the cement bone interface.
The devices as disclosed in the above mentioned Ling patents have been commercialized in the Zimmer CPT™ tapered highly polished stems. Other such stems include the Stryker Howmedica Exeter™ and the C-Stem™ (a DePuy product). These tapered highly polished stems have displayed clinical success, but have done so without the benefits of a collar. Because these designs lack consistent methods of proximal pressurization and centralization, cement mantle variability has been demonstrated. Meanwhile, collared stems such as the Charnley hip and its derivatives have also demonstrated excellent results, but do not have the additional advantage of controlled long-term subsidence.
Tapered highly polished stems have historically been designed without a proximal collar to prevent cement creep and long term, controlled subsidence within the cement mantle while simultaneously minimizing cement abrasion. Without the collar, however, the surgeon is intra-operatively challenged to position the stems both axially and rotationally. This malposition has been shown to limit the patient's range of motion by inducing improper leg length, inadequate lateral stem offset or non-anatomical version of the stem. Inadequate pressurization of the cement within the femoral canal has also been documented as a potential cause of improper cement technique.
Centralization of the stem within the stem mantle is also critical for success. Non-uniform or excessively thin cement mantles can induce high cement stresses and subsequent cracks that cause failure at the cement-stem-bone interfaces. Cement debris due to abrasions has also been shown to produce excessive third-body of polyethylene acetabular components as well as potentially induce osteolytic reactions and bone resorption that may lead to stem loosening. One single femoral stem that can singularly address these clinical issues is the intended solution.